The present study is undertaken to explore whether fine structural changes in cerebral cortex, not demonstrable by standard neuropathological methods, form the anatomical basis for the large proportion of cases of mental retardation in whom no gross developmental or destructive lesions of the brain are evident. In particular, synaptic and dendritic development in cerebral cortex is to be studied, utilizing both special histologic methods (the Golgi-Cox method for demonstration of dendritic arborization and the phosphotungstic acid method for synapses) and biochemical correlates of synaptic density (ganglioside sialic acid levels and activity of Na ion - K ion ATPase).